Kaposi sarcoma associated herpesvirus (KSHV) reprograms cells upon infection. One other outcome of this reprogramming is cellular transformation, such that KSHV-infected cells give rise to human cancer. KSHV causes cancers of two different cell lineages: B cells and endothelial cells. KSHV reprograms endothelial cells to adopt a lymphatic phenotype and induces hyperplasia of these lymphatic endothelial cells. This eventually leads to the emergence of Kaposi sarcoma (KS). KSHV reprograms B cells, though it is unclear exactly which subset and what the direct outcome is. Eventually this, too, leads to complete transformation and the emergence of signature, KSHV-associated, B cell cancers, i.e. a subclass of multicentric Castleman's disease (MCD) and primary effusion lymphoma (PEL). We hypothesize that this reprogramming is mediated, at least in part, by viral and cellular micro RNAs. We previously identified KS specific cellular miRNAs and we will investigate their regulation in KS biopsies and culture models. We generated in KSHV miRNA transgenic mice and will investigate their phenotypes with regard to B cell function and oncogene cooperation.